The present invention relates generally to systems and methods for determining characteristics of a formation surrounding a borehole. More particularly, the present invention relates to an acoustic logging tool that provides enhanced performance through use of programmable source waveforms.
Acoustic well logging is a well-developed art, and details of acoustic logging tools and techniques are set forth in A. Kurkjian, et al., xe2x80x9cSlowness Estimation from Sonic Logging Waveformsxe2x80x9d, Geoexploration, Vol. 277, pp. 215-256 (1991); C. F. Morris et al., xe2x80x9cA New Sonic Array Tool for Full Waveform Logging,xe2x80x9d SPE-13285, Society of Petroleum Engineers (1984); A. R. Harrison et al., xe2x80x9cAcquisition and Analysis of Sonic Waveforms From a Borehole Monopole and Dipole Source . . . xe2x80x9d SPE 20557, pp. 267-282 (September 1990); and C. V. Kimball and T. L. Marzetta, xe2x80x9cSemblance Processing of Borehole Acoustic Array Dataxe2x80x9d, Geophysics, Vol. 49, pp. 274-281 (March 1984), all of which are hereby incorporated by reference herein.
An acoustic logging tool typically includes an acoustic source (transmitter), and a set of receivers that are spaced several inches or feet apart. An acoustic signal is transmitted by the acoustic source and received at the receivers of the borehole tool which are spaced apart from the acoustic source. Measurements are repeated every few inches as the tool passes along the borehole.
The acoustic signal from source travels through the formation adjacent the borehole to the receiver array, and the arrival times and perhaps other characteristics of the receiver responses are recorded. Typically, compressional wave (P-wave), shear wave (S-wave), and Stoneley wave arrivals and waveforms are detected by the receivers and are processed. The processing of the data is often performed on the surface, although it may also be performed real time in the tool itself. Regardless, the information that is recorded is typically used to find formation characteristics such as formation slowness (the inverse of acoustic speed) and anisotropy, from which pore pressure, porosity, and other formation property determinations can be made. With some tools, the acoustic signals may even be used to image the formation.
Acoustic logging tools are used for both wireline logging and logging while drilling (LWD) applications. In wireline logging, a probe, or xe2x80x9csondexe2x80x9d, housing multiple logging tools is lowered into the borehole after some or all of the well has been drilled. The sonde is attached to a conductive wireline that carries power from the surface to the tools in the sonde, and that carries telemetry information to the surface. The sonde may be transported through the borehole by the wireline, or a separate transport means may be provided. For example, in xe2x80x9cpipe-conveyedxe2x80x9d logging, the sonde is mounted on a tubing string. The rigidity of the tubing string allows the sonde to be transported through highly deviated and horizontal boreholes.
The problem with obtaining downhole measurements via wireline is that the drilling assembly must be removed or xe2x80x9ctrippedxe2x80x9d from the drilled borehole before the desired borehole information can be obtained. This can be both time-consuming and extremely costly, especially in situations where a substantial portion of the well has been drilled. In this situation, thousands of feet of tubing may need to be removed and stacked on the platform (if offshore). Typically, drilling rigs are rented by the day at a substantial cost. Consequently, the cost of drilling a well is directly proportional to the time required to complete the drilling process. Removing thousands of feet of tubing to insert a wireline logging tool can be an expensive proposition.
As a result, there is a strong incentive to minimize the number of wireline logging trips. One way to do this involves collection of data during the drilling process. Designs for measuring conditions downhole including the movement and location of the drilling assembly contemporaneously with the drilling of the well have come to be known as xe2x80x9cmeasurement-while-drillingxe2x80x9d techniques, or xe2x80x9cMWDxe2x80x9d. Similar techniques, concentrating more on the measurement of formation parameters, commonly have been referred to as xe2x80x9clogging while drillingxe2x80x9d techniques, or xe2x80x9cLWDxe2x80x9d. While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
LWD tools are generally located as close to the drill bit as possible, so as to minimize the delay between reaching a formation and measuring its properties. When implemented as LWD tools, acoustic logging tools must overcome a number of obstacles to perform successfully. These obstacles include drilling noise, and acoustic properties of the thick tool body.
Accordingly, acoustic logging tools in both wireline and LWD applications have challenges to overcome.
Accordingly, there is disclosed herein an acoustic tool that provides a programmable source waveform. Numerous advantages may be achieved from the configurability of the source waveform. Notably, acoustic logs at multiple frequencies may be acquired with a single pass. The waveform may be frequency-adapted to maximize formation response and amplitude adapted for gain control. Numerous other potential advantages are also disclosed herein.
In one embodiment, the acoustic tool comprises: a controller, a digital-to-analog controller (DAC), an acoustic transducer, and a linear driver. The DAC converts a digital waveform from the controller into an analog waveform. The acoustic transducer converts an electrical signal into an acoustic signal. The linear driver receives the analog waveform from the DAC and responsively provides the electrical signal to the acoustic transducer. The electrical signal is proportional to the analog waveform. The tool may further include a memory for storing the digital waveform and/or software for generating the digital waveform.
The present invention further contemplates a logging method. The logging method comprises: (a) converting a digital waveform into an analog signal; (b) providing the analog signal to an acoustic transducer via a linear driver; and (c) producing an acoustic signal having a waveform that matches the digital waveform. The method may further include adapting the digital waveform to maximize a formation response and/or optimize the amplitude of a received signal.